1======================= 2The Framebuffer Console 3======================= 4 5The framebuffer console (fbcon), as its name implies, is a text 6console running on top of the framebuffer device. It has the functionality of 7any standard text console driver, such as the VGA console, with the added 8features that can be attributed to the graphical nature of the framebuffer. 9 10In the x86 architecture, the framebuffer console is optional, and 11some even treat it as a toy. For other architectures, it is the only available 12display device, text or graphical. 13 14What are the features of fbcon? The framebuffer console supports 15high resolutions, varying font types, display rotation, primitive multihead, 16etc. Theoretically, multi-colored fonts, blending, aliasing, and any feature 17made available by the underlying graphics card are also possible. 18 19A. Configuration 20================ 21 22The framebuffer console can be enabled by using your favorite kernel 23configuration tool. It is under Device Drivers->Graphics Support-> 24Console display driver support->Framebuffer Console Support. 25Select 'y' to compile support statically or 'm' for module support. The 26module will be fbcon. 27 28In order for fbcon to activate, at least one framebuffer driver is 29required, so choose from any of the numerous drivers available. For x86 30systems, they almost universally have VGA cards, so vga16fb and vesafb will 31always be available. However, using a chipset-specific driver will give you 32more speed and features, such as the ability to change the video mode 33dynamically. 34 35To display the penguin logo, choose any logo available in Graphics 36support->Bootup logo. 37 38Also, you will need to select at least one compiled-in font, but if 39you don't do anything, the kernel configuration tool will select one for you, 40usually an 8x16 font. 41 42GOTCHA: A common bug report is enabling the framebuffer without enabling the 43framebuffer console. Depending on the driver, you may get a blanked or 44garbled display, but the system still boots to completion. If you are 45fortunate to have a driver that does not alter the graphics chip, then you 46will still get a VGA console. 47 48B. Loading 49========== 50 51Possible scenarios: 52 531. Driver and fbcon are compiled statically 54 55 Usually, fbcon will automatically take over your console. The notable 56 exception is vesafb. It needs to be explicitly activated with the 57 vga= boot option parameter. 58 592. Driver is compiled statically, fbcon is compiled as a module 60 61 Depending on the driver, you either get a standard console, or a 62 garbled display, as mentioned above. To get a framebuffer console, 63 do a 'modprobe fbcon'. 64 653. Driver is compiled as a module, fbcon is compiled statically 66 67 You get your standard console. Once the driver is loaded with 68 'modprobe xxxfb', fbcon automatically takes over the console with 69 the possible exception of using the fbcon=map:n option. See below. 70 714. Driver and fbcon are compiled as a module. 72 73 You can load them in any order. Once both are loaded, fbcon will take 74 over the console. 75 76C. Boot options 77 78 The framebuffer console has several, largely unknown, boot options 79 that can change its behavior. 80 811. fbcon=font:<name> 82 83 Select the initial font to use. The value 'name' can be any of the 84 compiled-in fonts: 10x18, 6x10, 6x8, 7x14, Acorn8x8, MINI4x6, 85 PEARL8x8, ProFont6x11, SUN12x22, SUN8x16, TER16x32, VGA8x16, VGA8x8. 86 87 Note, not all drivers can handle font with widths not divisible by 8, 88 such as vga16fb. 89 90 912. fbcon=map:<0123> 92 93 This is an interesting option. It tells which driver gets mapped to 94 which console. The value '0123' is a sequence that gets repeated until 95 the total length is 64 which is the number of consoles available. In 96 the above example, it is expanded to 012301230123... and the mapping 97 will be:: 98 99 tty | 1 2 3 4 5 6 7 8 9 ... 100 fb | 0 1 2 3 0 1 2 3 0 ... 101 102 ('cat /proc/fb' should tell you what the fb numbers are) 103 104 One side effect that may be useful is using a map value that exceeds 105 the number of loaded fb drivers. For example, if only one driver is 106 available, fb0, adding fbcon=map:1 tells fbcon not to take over the 107 console. 108 109 Later on, when you want to map the console the to the framebuffer 110 device, you can use the con2fbmap utility. 111 1123. fbcon=vc:<n1>-<n2> 113 114 This option tells fbcon to take over only a range of consoles as 115 specified by the values 'n1' and 'n2'. The rest of the consoles 116 outside the given range will still be controlled by the standard 117 console driver. 118 119 NOTE: For x86 machines, the standard console is the VGA console which 120 is typically located on the same video card. Thus, the consoles that 121 are controlled by the VGA console will be garbled. 122 1234. fbcon=rotate:<n> 124 125 This option changes the orientation angle of the console display. The 126 value 'n' accepts the following: 127 128 - 0 - normal orientation (0 degree) 129 - 1 - clockwise orientation (90 degrees) 130 - 2 - upside down orientation (180 degrees) 131 - 3 - counterclockwise orientation (270 degrees) 132 133 The angle can be changed anytime afterwards by 'echoing' the same 134 numbers to any one of the 2 attributes found in 135 /sys/class/graphics/fbcon: 136 137 - rotate - rotate the display of the active console 138 - rotate_all - rotate the display of all consoles 139 140 Console rotation will only become available if Framebuffer Console 141 Rotation support is compiled in your kernel. 142 143 NOTE: This is purely console rotation. Any other applications that 144 use the framebuffer will remain at their 'normal' orientation. 145 Actually, the underlying fb driver is totally ignorant of console 146 rotation. 147 1485. fbcon=margin:<color> 149 150 This option specifies the color of the margins. The margins are the 151 leftover area at the right and the bottom of the screen that are not 152 used by text. By default, this area will be black. The 'color' value 153 is an integer number that depends on the framebuffer driver being used. 154 1556. fbcon=nodefer 156 157 If the kernel is compiled with deferred fbcon takeover support, normally 158 the framebuffer contents, left in place by the firmware/bootloader, will 159 be preserved until there actually is some text is output to the console. 160 This option causes fbcon to bind immediately to the fbdev device. 161 1627. fbcon=logo-pos:<location> 163 164 The only possible 'location' is 'center' (without quotes), and when 165 given, the bootup logo is moved from the default top-left corner 166 location to the center of the framebuffer. If more than one logo is 167 displayed due to multiple CPUs, the collected line of logos is moved 168 as a whole. 169 1708. fbcon=logo-count:<n> 171 172 The value 'n' overrides the number of bootup logos. 0 disables the 173 logo, and -1 gives the default which is the number of online CPUs. 174 175C. Attaching, Detaching and Unloading 176 177Before going on to how to attach, detach and unload the framebuffer console, an 178illustration of the dependencies may help. 179 180The console layer, as with most subsystems, needs a driver that interfaces with 181the hardware. Thus, in a VGA console:: 182 183 console ---> VGA driver ---> hardware. 184 185Assuming the VGA driver can be unloaded, one must first unbind the VGA driver 186from the console layer before unloading the driver. The VGA driver cannot be 187unloaded if it is still bound to the console layer. (See 188Documentation/driver-api/console.rst for more information). 189 190This is more complicated in the case of the framebuffer console (fbcon), 191because fbcon is an intermediate layer between the console and the drivers:: 192 193 console ---> fbcon ---> fbdev drivers ---> hardware 194 195The fbdev drivers cannot be unloaded if bound to fbcon, and fbcon cannot 196be unloaded if it's bound to the console layer. 197 198So to unload the fbdev drivers, one must first unbind fbcon from the console, 199then unbind the fbdev drivers from fbcon. Fortunately, unbinding fbcon from 200the console layer will automatically unbind framebuffer drivers from 201fbcon. Thus, there is no need to explicitly unbind the fbdev drivers from 202fbcon. 203 204So, how do we unbind fbcon from the console? Part of the answer is in 205Documentation/driver-api/console.rst. To summarize: 206 207Echo a value to the bind file that represents the framebuffer console 208driver. So assuming vtcon1 represents fbcon, then:: 209 210 echo 1 > /sys/class/vtconsole/vtcon1/bind - attach framebuffer console to 211 console layer 212 echo 0 > /sys/class/vtconsole/vtcon1/bind - detach framebuffer console from 213 console layer 214 215If fbcon is detached from the console layer, your boot console driver (which is 216usually VGA text mode) will take over. A few drivers (rivafb and i810fb) will 217restore VGA text mode for you. With the rest, before detaching fbcon, you 218must take a few additional steps to make sure that your VGA text mode is 219restored properly. The following is one of the several methods that you can do: 220 2211. Download or install vbetool. This utility is included with most 222 distributions nowadays, and is usually part of the suspend/resume tool. 223 2242. In your kernel configuration, ensure that CONFIG_FRAMEBUFFER_CONSOLE is set 225 to 'y' or 'm'. Enable one or more of your favorite framebuffer drivers. 226 2273. Boot into text mode and as root run:: 228 229 vbetool vbestate save > <vga state file> 230 231 The above command saves the register contents of your graphics 232 hardware to <vga state file>. You need to do this step only once as 233 the state file can be reused. 234 2354. If fbcon is compiled as a module, load fbcon by doing:: 236 237 modprobe fbcon 238 2395. Now to detach fbcon:: 240 241 vbetool vbestate restore < <vga state file> && \ 242 echo 0 > /sys/class/vtconsole/vtcon1/bind 243 2446. That's it, you're back to VGA mode. And if you compiled fbcon as a module, 245 you can unload it by 'rmmod fbcon'. 246 2477. To reattach fbcon:: 248 249 echo 1 > /sys/class/vtconsole/vtcon1/bind 250 2518. Once fbcon is unbound, all drivers registered to the system will also 252become unbound. This means that fbcon and individual framebuffer drivers 253can be unloaded or reloaded at will. Reloading the drivers or fbcon will 254automatically bind the console, fbcon and the drivers together. Unloading 255all the drivers without unloading fbcon will make it impossible for the 256console to bind fbcon. 257 258Notes for vesafb users: 259======================= 260 261Unfortunately, if your bootline includes a vga=xxx parameter that sets the 262hardware in graphics mode, such as when loading vesafb, vgacon will not load. 263Instead, vgacon will replace the default boot console with dummycon, and you 264won't get any display after detaching fbcon. Your machine is still alive, so 265you can reattach vesafb. However, to reattach vesafb, you need to do one of 266the following: 267 268Variation 1: 269 270 a. Before detaching fbcon, do:: 271 272 vbetool vbemode save > <vesa state file> # do once for each vesafb mode, 273 # the file can be reused 274 275 b. Detach fbcon as in step 5. 276 277 c. Attach fbcon:: 278 279 vbetool vbestate restore < <vesa state file> && \ 280 echo 1 > /sys/class/vtconsole/vtcon1/bind 281 282Variation 2: 283 284 a. Before detaching fbcon, do:: 285 286 echo <ID> > /sys/class/tty/console/bind 287 288 vbetool vbemode get 289 290 b. Take note of the mode number 291 292 b. Detach fbcon as in step 5. 293 294 c. Attach fbcon:: 295 296 vbetool vbemode set <mode number> && \ 297 echo 1 > /sys/class/vtconsole/vtcon1/bind 298 299Samples: 300======== 301 302Here are 2 sample bash scripts that you can use to bind or unbind the 303framebuffer console driver if you are on an X86 box:: 304 305 #!/bin/bash 306 # Unbind fbcon 307 308 # Change this to where your actual vgastate file is located 309 # Or Use VGASTATE=$1 to indicate the state file at runtime 310 VGASTATE=/tmp/vgastate 311 312 # path to vbetool 313 VBETOOL=/usr/local/bin 314 315 316 for (( i = 0; i < 16; i++)) 317 do 318 if test -x /sys/class/vtconsole/vtcon$i; then 319 if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \ 320 = 1 ]; then 321 if test -x $VBETOOL/vbetool; then 322 echo Unbinding vtcon$i 323 $VBETOOL/vbetool vbestate restore < $VGASTATE 324 echo 0 > /sys/class/vtconsole/vtcon$i/bind 325 fi 326 fi 327 fi 328 done 329 330--------------------------------------------------------------------------- 331 332:: 333 334 #!/bin/bash 335 # Bind fbcon 336 337 for (( i = 0; i < 16; i++)) 338 do 339 if test -x /sys/class/vtconsole/vtcon$i; then 340 if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \ 341 = 1 ]; then 342 echo Unbinding vtcon$i 343 echo 1 > /sys/class/vtconsole/vtcon$i/bind 344 fi 345 fi 346 done 347 348Antonino Daplas <adaplas@pol.net> 349